In high-speed optical signal transmission, distortion of waveform of digital optical signal occurs due to various factors, and particularly due to dispersion of the transmission media such as the optical fiber. The amount of waveform distortion normally increases when the optical source of signal is accompanied by large amount of unwanted and usually unmanageable frequency shift, also known as chirp, across its transmitting waveform. This is due to the fact that different frequency components of the optical signal, in particular when being loaded with undesirable frequency shift, will propagate across a fiber media at different speed, causing edges of the waveform to gradually smear and eventually become unrecognizable at its intended destination by an optical receiver.
This phenomenon manifests itself most severely in directly modulated laser signal. In a directly modulated laser signal, there exists large amount of un-manageable chirps across its waveform. The resulting waveform distortion causes inter-symbol-interference (ISI) in digital optical signal transmission, creates receiver sensitivity degradation, and ultimately limits the reachable transmission distance for the optical signal.
For example, FIG. 1 illustrates a directly modulated optical signal source as is known in the art. The optical signal source normally employs a directly modulated laser diode (DM-LD) 11. Optical data signal output from laser diode 11 is coupled to optical fiber 10 for transmission. During operation, laser diode 11 is normally biased at around its lasing threshold point 30 which is a point in the L-I curve (optical power versus drive current) between linear operational region 32 and laser shut off region 31 as being illustratively shown in FIG. 1. An electrical data signal 21 is applied to modulate laser diode 11, which is normally superimposed onto the bias current at bias point 30 of laser diode 11. Laser diode 11 thus converts input electrical data signal 21 into an output optical data signal 12. Because laser diode 11 is being turned on-and-off around its threshold point 30 for lasing to generate optical data signal 12, a large amount of unwanted and usually unmanageable chirps or frequency shifts may be created, particularly around the “0” state of optical data signal 12. The chips ride with optical data signal 12 and propagate down fiber link 10 with optical data signal 12 causing waveform distortion. In addition, in a directly modulated laser source, conversion of electrical data signal 21 to optical data signal 12 follows the L-I curve of laser diode 11. Since the conversion curve is not exactly linear (31, 32) around threshold point 30, the optical data signal 12 may not truly represent the waveform of electrical data signal 21, causing waveform being distorted at the transmitting side as well.
Due to the existence of large amount of unwanted and un-manageable chirp, directly modulated laser signal sources are generally considered as not being suitable for high-speed, and/or long distance transmission, and therefore are usually used in relatively low speed, for example, 2.5 Gb/s or below data communications, and/or in signal transmission of relatively short distances.
In order to achieve high-speed, for example 10 Gb/s and beyond, optical signal transmission and reach long distances, the state-of-art technology is to modulate a CW (continuous wave) optical signal by using an external modulator. The type of external modulators may include, for example, LiNbO3 or III-V semiconductor based Mach-Zehnder type modulator and electro-optic absorption type modulator. These types of modulators, in particular a LiNbO3 Mach-Zehnder modulator with the high cost associated therewith, have been shown to be able to effectively manage and in certain cases tailor the frequency chirp to meet particular optical signal transmission needs.